(1) Field of the Invention
The present invention relates to a light intensity control apparatus for controlling a light intensity of a light-emitting element and, more particularly, to a light intensity control apparatus suitably used for an optical printer or the like.
(2) Related Background Art
As laser light intensity control in a conventional laser beam printer, an APC (Auto Power Control) scheme is used, and is employed in most of the existing laser beam printers.
In the APC scheme, a laser beam is emitted and is received by a light-receiving element. The light intensity of the received laser beam is photo-electrically converted into an electrical amount. The electrical amount is compared with a predetermined reference value. As a result of comparison, if a decrease in laser light intensity is detected, control is made so that a laser drive current is increased to increase a light intensity; otherwise, the laser drive current is decreased to decrease a light intensity. This control will be referred to as Laser APC hereinafter.
In the recent laser beam printers, the Laser APC is executed at the beginning of printing and at a paper interval during a printing operation. The paper interval means a non-image region offset from an image forming region on a drum in a rotating direction of a drum (sub-scan direction of the laser).
In this scheme, more specifically, an APC-START request is issued from a central processing unit of the laser beam printer at the paper interval, so that the Laser APC is executed once.
When the APC-START request is issued, the laser drive current is temporarily cleared to 0 mA, and is then gradually increased stepwise. In this case, an electrical amount from the light-receiving element is compared with a predetermined value in each step. When the electrical amount from the light-receiving element coincides with the predetermined value, an increase in laser drive current is stopped, a forcible emission of the laser is canceled, and the value is held until the next APC-START request is issued. Therefore, in this scheme, the light intensity of the laser is increased stepwise, and when a predetermined light intensity is reached, the laser is deenergized. Therefore, the laser is continously turned on to scan several lines to several tens of lines on a photosensitive drum. The Laser APC by this scheme will be referred to as paper interval continuous Laser APC hereinafter.
In another Laser APC scheme, when raster scan of a laser scans a region other than a photosensitive drum surface, the Laser APC is executed. Note that a scan region by the raster scan and other than the photosensitive drum surface, i.e., a region offset from an image forming region on the drum in the axial direction of the drum (main scan direction of the laser) will be referred to as a non-drum region hereinafter.
Image formation of the laser beam printer is executed in units of lines by raster scan. Each line is established as an image by outputting image data in synchronism with a horizontal sync signal (to be referred to as a beam detect (BD) signal hereinafter).
In order to obtain a BD signal, the laser beam printer turns on the laser at line intervals (a laser turn-on instruction signal will be referred to as an unblanking (UNBL) signal hereinafter).
The Laser APC is executed on the non-drum region in synchronism with the UNBL signals generated at the line intervals. This scheme will be referred to as non-drum region Laser APC.
With the above-mentioned Laser APC schemes, the laser is controlled to always have a stable laser light intensity during execution of image formation, thus improving image quality.
More specifically, a laser is gradually degraded during use, and then stops emission. Although the degree of degradation varies depending on individual lasers, each laser will be dead sooner or later, and light intensity control by the Laser APC is disabled. When the Laser APC is disabled and a predetermined light intensity cannot be obtained, it is determined that the laser is dead and replaced.
However, in the paper interval continuous Laser APC, since a laser beam is always radiated on the photosensitive drum during execution of the Laser APC, a developing agent (to be referred to as a toner hereinafter) is undesirably attached to the radiated portion. As a result, when a transfer device is of a dielectric belt type or a transfer roller type which is brought into contact with the photosensitive drum or approaches very close to it, the toner attached to the photosensitive drum is attached to the transfer device.
There is no problem if the circumferential length of the dielectric belt or transfer roller is the same as that of the photosensitive drum. However, they normally have the different circumferential lengths. Therefore, when the first paper interval continuous Laser APC is executed, even if a laser radiation position is located at the paper interval, it often returns to a transfer point during image formation. Thus, the lower surface of a print sheet is contaminated.
When a double-side printing operation is performed, contamination of the lower surface of the print sheet causes a decisive drawback.
Even if a print sequence is executed or the arrangement of the transfer device is modified so that an interval of a toner portion attached to the transfer device by the paper interval continuous Laser APC must return to the paper interval of a print sheet, a condition for realizing this considerably restricts the printing operation or increases cost.
The laser beam printer is normally controlled by an external image forming apparatus which outputs an image, and executes a printing operation. However, the image forming apparatus does not unilaterally output an instruction command to the laser beam printer but must output image data in synchronism with a BD signal output from the laser beam printer in units of lines when an image is printed.
Therefore, for the laser beam printer, the BD signal is an important signal for the image forming apparatus. When the image forming apparatus causes the laser beam printer to execute a printing operation, it often uses one pulse of a BD signal per line as a coefficient of a counter in vertical control. Although the BD signal is indispensable during image printing, if the BD signal is output at the paper interval, it is convenient for the next print control, and control of the laser beam printer can be facilitated.
However, since the current laser beam printer temporarily clears and rises a laser light intensity to execute paper interval continuous Laser APC, light emission of the laser is interrupted. During execution of the paper interval continuous Laser APC, the BD signal cannot be detected, and a BD signal count at the paper interval cannot be guaranteed. For this reason, the image forming apparatus must control timings using, e.g., a timer at the paper interval, resulting in complicated control.
In a method of executing the Laser APC on the non-drum region, since a time corresponding to the non-drum region is about 100 .mu.s to 200 .mu.s in a normal laser beam printer, it is impossible in view of time to perform similar control to the above-mentioned paper interval continuous Laser APC during this time period. Even if an IC capable of performing ultra high-speed processing is used, this results in a considerable increase in cost, and is not practical. Therefore, for one non-drum region, control including light intensity measurement, comparison, and light intensity correction corresponding to one step in the paper interval continuous Laser APC can only be performed. Therefore, in order to control the light intensity to a predetermined value by this Laser APC, a period of several lines to several tens of lines is required. When correction is restarted from a level of several % to several tens of % of a predetermined light intensity, an image density is decreased during several lines to several tens of lines until the predetermined light intensity is reached, and density nonuniformity occurs, thus degrading image quality.
Even if control is made to perform light intensity control for one step during one non-drum region, when a resolution per step is rough, a density varies in units of lines due to a difference in light intensity before and after correction, and density nonuniformity becomes conspicuous.
A resolution per step will be briefly described below.
A laser light intensity is normally controlled by a laser drive current amount. The laser drive current is determined by a voltage regulating circuit controlled by an output voltage of a D/A converter. More specifically, the resolution of the D/A converter determines the resolution of the laser light intensity. In general, since a maximum drive current of a semiconductor laser is set at 120 mA, a current of 140 mA is considered as a maximum setup value taking variations of circuit constants into account. When the laser begins to emit light, it emits light at a light intensity determined by a slope efficiency mW/mA indicating a change in light intensity per unit current. However, the slope efficiency varies depending on individual lasers, and has a margin of about 0.1 to 0.6 mW/mA.
Therefore, if a 10-bit D/A converter, i.e., having a resolution of 1,023 steps is used, a current value per step is: EQU 140 mA.div.1,023 steps.apprxeq.0.137 mA/step
Therefore, EQU 0.137 mA/step.times.0.6 mW/mA=0.082 mW/step
However, in the laser beam printer, since a minimum value of an available laser light intensity is about 1 mW, then EQU (0.082 mW/step.div.1 mW).times.100=8.2%/step
Therefore, a variation rate of the light intensity per step is 8.2%.
In the laser beam printer, the variation rate causing density nonuniformity is generally 5%. Therefore, when the Laser APC is executed every page, density nonuniformity in a page does not occur. However, if the Laser APC is executed every line, density nonuniformity occurs.
As described above, the non-drum region Laser APC has a serious problem of a limited processing time, and a degree of stability of light intensity correction, and is not established yet as a control method which can be employed by the laser beam printer.
As described above, in the non-drum region Laser APC, since the non-drum region is detected on the basis of the UNBL signal, the BD signal for obtaining a proper pulse timing of the UNBL signal is an indispensable condition.
However, this BD signal is generated when a laser beam is radiated on a light-receiving element. When a laser beam rises, it starts from the state of a light intensity of 0, and the UNBL signal cannot be obtained. Therefore, control at a rise time of the non-drum region Laser APC on the basis of the UNBL signal cannot be executed.
When the control at the rise time of the Laser APC is performed in a continuous emission state regardless of the UNBL signal, a laser beam is radiated on an image forming region of the photosensitive drum surface, as described above, and the lower surface of a print sheet is contaminated.
A drive current at which the laser can emit light is set, and can be started from this value to rise a light intensity to a desired value at the non-drum region. However, since a threshold current Ith of the presently used laser has a margin of 20 mA to 60 mA due to variations in characteristics, a uniform setup value cannot be determined.
If the setup vallue is set at the threshold current Ith of 160 mA, when a calculation is made to have an average value of the slope efficiency .eta. of 0.3 mW/mA, the light intensity for a laser having the threshold current Ith=20 mA is given by: ##EQU1## Since the present rated light intensity is 5 mW, some lasers must start the Laser APC while exceeding the rated value, and may be broken.
Therefore, a preferable setup value cannot be determined. If a laser to be used is limited to use a uniform setup value, the cost of the apparatus is considerably increased.
As described above, an image recording apparatus which forms and visualizes an electrostatic latent image on a photosensitive drum by a light beam radiated from a semiconductor light-emitting element, e.g., a laser has been proposed.
In the apparatus of this type, in order to suppress a variation in light output due to heat dissipation of a semiconductor light-emitting element, or the like, so-called APC is performed parallel to image recording processing, such that a light output of the semiconductor light-emitting element is received by a photosensor, e.g., a photodiode, and a laser drive current is controlled to obtain a constant light output.
In an LED printer using an LED array as semiconductor light-emitting elements, similar APC is performed.
The APC is normally performed once per page printing, i.e., between two adjacent pages.
In particular, in a continuous printing mode, a light-emitting operation for the APC is performed between two adjacent images to be printed. In this case, in the LED printer or the laser beam printer, since a surface portion of a photosensitive drum irradiated with a laser beam for, e.g., several msec (non-image forming region which does not normally serve as an image forming region) is developed in solid black, an extra toner as a developing agent is consumed by processing in addition to normal image recording processing. The developed toner on the photosensitive drum is directly recovered by a cleaner without being transferred onto a recording medium such as a recording sheet.
Therefore, in a cartridge type developing unit, a developing capacity and a recovery volume are determined in consideration of unnecessary toner consumption or a toner recovery capacity by the APC. However, when the developing unit is rendered compact to meet recent demand for a compact apparatus, the developing capacity and recovery volume disturb compactness of the apparatus.
The conventional cleaner device is arranged to be able to recover a residual toner which is not transferred onto a sheet. In development caused by APC exposure, the developed toner is directly recovered by the cleaner without being transferred onto a sheet. Therefore, a recovery load of the cleaner is large, and a cleaning error tends to occur.
When a transfer process is realized by a roller or belt transfer scheme, a roller or belt is in contact with the photosensitive drum. Thus, the roller or belt is contaminated with a toner on a non-paper portion (i.e., a region developed for the APC).
The present applicant proposed apparatuses for controlling light intensities of light-emitting elements in U.S. Pat. No. 4,201,994, U.S. Pat. No. 4,443,695, U.S. Ser. No. 087,707 (filed Aug. 21, 1987) and U.S. Ser. No. 232,168 (filed Aug. 15, 1988). However, a further improvement has been demanded.